Document printing in the home and in the office has evolved to incorporate color into a large number of documents. The use of color in documents has created problems in the area of printing color text, that extends to other small features in the document images. The readability of colored text is strongly influenced by the choice of color, not only due to the visual contrast between text and background, but also due to the printing process incorporating a finite set of colorants at a small number of defined local density levels.
Color in documents is the result of a combination of a limited set of colorants over a small area, in amounts selected to integrate to a desired color response. This is accomplished in many printing devices by reproducing separations of the image, where each separation provides varying amounts of a single primary colorant. When combined together with other separations, the result is a full color image.
For color images, a plurality of color separations are combined. Each color separation corresponds to a single colorant, and may be defined by a number of gray levels in excess of the capability of the printer. In such cases, each of the color separations is reduced from the input number of levels to the smaller output number of levels reproducible by the printer. The multiple color separations are combined together at printing to yield the final color print. Commonly, color documents are formed using cyan, magenta and yellow colorants or cyan, magenta, yellow and black colorants. A larger number, or alternative colorants may also be used. A color image may also be represented as sets of image planes representing quantities other than colorants. For instance, the color planes could represent parameters within a representation of color, such as L*, a* and, b* which describe lightness and two aspects of chromaticity, respectively.
In printing documents, the desired gray level over an area is commonly achieved by halftoning, where image gray level variation is represented by placing greater or lesser numbers of ON pixels in a discrete area of the image. In one halftoning method, known as dithering or screening, over a given area having a number of gray separation pixels therein, a value representing the gray level of each separation pixel of an array of gray separation pixels within the area is compared to one of a set of preselected thresholds as taught, for example, in U.S. Pat. No. 4,149,194 to Holladay. For an area where the image is gray, some of the thresholds within the dither matrix will be exceeded, i.e., the image value at that specific location is larger than the value stored in the dither matrix for that same location, while others are not. In the binary case, the image pixels or cell elements for which the thresholds are exceeded might be printed as a maximum colorant value, while the remaining separation pixels are allowed to remain white, dependent on the actual physical quantity described by the data. The described halftoning method produces an output pattern that is periodic or quasi-periodic in the spatial coordinates.
Halftoning creates problems in color document reproduction where color text and other small image features are used. One constant trade-off in color printing is between spatial resolution and color resolution. In order to reproduce a given color accurately, a comparatively large halftone cell has to be used. Large halftone cells allow more gray levels of each color, and therefore, more colors. However, the effect of this large cell is that small details can no longer be accurately rendered. The large halftone cell structure becomes visible to the eye. An impact of this phenomenon is that small colored text objects become illegible. If on the other hand, a comparably small halftone cell is chosen, the color of graphics and images is no longer accurately represented. However, if the halftones cells used to create the color are changed to smaller cells, the choice of colors becomes smaller as well, making it unlikely that the correct color can be obtained.
The problem of small color text reproduction extends to many other small features, including for example, structures in maps and other detailed illustrations, particular when such structures are in color. One notable example is the representation of a certain type of road on a map as a solid, thin colored line. If the road is improperly halftoned, the road may appear "dashed" and the original image intent is lost.
All of the references cited herein are incorporated by reference for their teachings.